34 replies to this topic

[JetBLICK]

I spoke to bromsgrove engine services today about tuning a 1071 's' motor..... they're good chaps there, do lots of work with mini engines, and the guy has a racer, so i'm sure he knows what he's talking about. But i was talking to him about boring a cooper 's' block and he didn't recommend going over 60thou, i was after biggest cc's i could out of a 1071 - so a 73.5mm bore, however he said teh cooper 's' blocks dont respond to it very well....

Just wanted to ask if anyone knew why?

[The Matt]

Now, I have asked this before and I can't remember what the answer was!

That's no help at all!

[Jammy]

Because boring out a 1071 block is sacrilegious!!

[JetBLICK]

well lets say in a hypothetical situation, the block is already at +60 thou and may need a rebore if it cant be honed and i dont really fancy sleeving it back to standard unless i really need to - hypothetically of course

[The Matt]

Because boring out a 1071 block is sacrilegious!!

mmmmm, sacrelicious

Anyway, is the 1071 a derivative of a small bore block anyway?

[JetBLICK]

no no no no, the 1071 is a big bore motor with a short stroke - basically a 1275 with a 68mm stroke

its the engine that paddy won the monte in and only 4000'odd where made for the road

Edited by JetBLICK, 25 April 2007 - 01:17 PM.

[The Matt]

Ja, but weren't the early 1275 engines based around the small bore block? Rather than the likes of the 1275GT block being actually designed to be that bore. I thought that was why the Cooper engines also had smaller big ends than the larger bore blocks????

I'll get my coat!

[Jammy]

I would assume its something like the 1071 is a very high reving engine anyway, and it'll be an A series, so no strengthening ribs, so it may not last long if you don't leave a lot metal between the bores!?

[Big_Adam]

....errmm.....if it's basicaly a 1275 and you want bigger.....why not put a 1275 crank it in?

[biggav]

....errmm.....if it's basicaly a 1275 and you want bigger.....why not put a 1275 crank it in?

I think its basically a 998 bored out to 1275 piston size and short stroked, the block deck height is also reduced to correct the compression ratio.... if you put a 1275 crank, the pistons would then come out the top of the block due to the increased stroke....

If you started with a 1275 engine, i think the blocks are stronger and the oil chambers and bits and bobs are more out of the way to enable bigger over bores... you could then bore the 1275 out to 73.5 and reduce the stroke by getting a new crank and lowering the deck height...

I think Vizards book recommends not boring the a block beyond a certain amount as there is a huge failure pottential.

Edited by biggav, 25 April 2007 - 01:53 PM.

[JetBLICK]

....errmm.....if it's basicaly a 1275 and you want bigger.....why not put a 1275 crank it in?

Dont swear at me! oh deary deary me....i think you need to understand the awsomeness of a 1071 before we let you back in the room

Let me try and put it simply... take a seat...

Apart from the 970 and 1071 engines all a-series engines are 'over stroked' buy that i mean the stroke of the crank is longer than the bore. Engines like this are better for fuel economy and produce more torque lower in the rev range. Buy having a short stroke or square engine like the 1071 or 970, where the bore is bigger than the stroke or the same size you create an engine that will rev much more freely. And more revs make for a better race engine. I cant be arsed to sit hear and write and essay about it and explain why, but consider this. Nearly all modern engines are short stroke or square, motorbike engines, that rev their nutts off, are very short stroke.

The downside of a short stroker is that they don't produce their torque till higher in the rev range. It depends entirely on how you like to drive tho, for a daily driver ur 1275-1380 is great, it'll pull away from the lights with ease and u'll get good fuel economy. But if u like racing around, and can put up with it, a short stroke is the way you want to go. With more rev's you can have a shorter final drive, which means better acceleration and response when you bury you're right foot.

Theres loads of topics on this do a search, there are some right gems in there.

Dan

Edited by JetBLICK, 25 April 2007 - 02:24 PM.

[The Matt]

I think its basically a 998 bored out to 1275 piston size and short stroked, the block deck height is also reduced to correct the compression ratio.... if you put a 1275 crank, the pistons would then come out the top of the block due to the increased stroke....

I think Vizards book recommends not boring the a block beyond a certain amount as there is a huge failure pottential.

Exactly, this is how I thought it was. The 1071 started off life as a small bore block, but was overbored to 10.6mm or whatever it was, then the stroke was altered. I think that is why the engine tuners wouldn't recommend taking them out to 73.5mm.

[Jammy]

So what bore is the block currently? +60? I've already stated why I think the block doesn't take to large bores but your best waiting for someone like Dan to see if he knows.

A 1071 block with twin carbs would be awesome, the twins should make up for the lack of torque and the responsiveness of the twins coupled with the rev-ability of the engine would be superb!

[JetBLICK]

I think its basically a 998 bored out to 1275 piston size and short stroked, the block deck height is also reduced to correct the compression ratio.... if you put a 1275 crank, the pistons would then come out the top of the block due to the increased stroke....

I think Vizards book recommends not boring the a block beyond a certain amount as there is a huge failure pottential.

Exactly, this is how I thought it was. The 1071 started off life as a small bore block, but was overbored to 10.6mm or whatever it was, then the stroke was altered. I think that is why the engine tuners wouldn't recommend taking them out to 73.5mm.

nar nar nar.... well ur right in some respects.... The 1071 came first, the 1275 was developed from it and the 970 after that. But the cooper s engines are not part of the small bore family. The bore of the 3 motors was 70.6mm, and the cranks where 68 81 and 61 respectivly. The big bore blocks where radically different to the small bores tho, the mains bearings are a dead givaway, being 2" dia - hense why you cant fit 1275's cranks in 998's.

[biggav]

i started a topic the other week about doing a very big bore short stroker... maybe even an 8 port head, just to see what it'll do... the thing is finding a gearbox suited to the job.... i'm not the kind of person who wants to rebuild an engine everyfew thousand miles just to make sure... so i'm sticking with the 1380 and giggle gas for the moment.